1. Field of the Invention
This invention relates to the field of conductors. More specifically, it is Nb3Sn superconducting wire having enhanced mechanical strength and critical current density compared with the unstrengthened counterpart if the heat treatment was the same for both wires.
2. Description of the Related Art
FIGS. 1 and 2 illustrate in simplified form a known method of creating composite wires. In FIG. 1, a bundle of wires 14 is placed within a hollow tube 12 to create precursor assembly 10.
In FIG. 2, precursor assembly 10 is passed through drawing die 16 to form drawn composite wire 18. The materials used to form the precursor assembly are reasonably ductile, so that when the assembly is forced through the drawing die the materials will codeform to eliminate voids and create a unified whole.
The actual processes known in the prior art are considerably more complex. For example, wires 14 shown in FIG. 1 may actually be composite wires including several dissimilar materials. These composite wires may have been formed by a prior drawing process. However, the simplified representations of FIGS. 1 and 2 ably serve to familiarize the reader with one of the known fabrication techniques in the field of superconducting wires.
Niobium-tin compounds (typically Nb3Sn) are commonly used in creating superconducting wires. Such wires are useful in high-field magnets. However, during operation the conductors employed are subjected to substantial mechanical stress due to the electro-magnetic force (Lorentz forces). Niobium-tin superconductors are sensitive to mechanical strain, so the electro-magnetic force will dramatically degrade the superconducting properties, particularly the critical current (“critical current” means the maximum current the wire can carry without losing its superconducting properties) of the compound.
The second issue is that the materials need relatively long heat treatment time in order to have fully reacted Nb3Sn conductors. This results in not only consumption of energies but also complex of the fabrications.
The prior solution to the first problem has been to reinforce the niobium-tin compounds with other materials. Copper-Niobium composite, tantalum, Nb—Ti—Cu compound and Cu—Ni—Nb—Ti compound have been used to increase the mechanical strength of Nb3Sn, with varying degrees of success. However, all these attempts share a common drawback: The inclusion of the reinforcing material reduces the fraction of Copper stabilizer or Non-Copper area. The reduction of the Copper stabilizer decreases the wire's overall stability and limits the practical applications of the superconductor composite wires, and the reduction of Non-Copper area decreases the critical current value.
A better solution would be to strengthen the niobium-tin wires in a way, which does not compromise the superconducting properties. The current invention presents a method which can achieve this objective. In fact, the current invented approach enhances both the mechanical strength and the crucial current of the superconductor composite wires.
The current methods also shorten the heat treatment time so that energy will be saved and fabrication procedure is simplified.
Prior fabrication approaches involve evacuation and sealing of the composite wires in a separate fabrication step in order to enhance the bounding between different components. The current approach eliminates the extra sealing step and the sealing is achieved during the co-deformation steps.